The goal of this project is to develop optical coherence domain reflectometry (OCDR) for accurate image guided placement of treatment probes in deep-brain structures. Refined guidance would greatly enhance the effectiveness and safety of deep-brain stimulation (DBS) with implanted electrodes. DBS is an FDA-approved treatment for Parkinson's disease and essential tremor that provides long-term relief of symptoms when medications are inadequate. It is also a promising therapy for intractable dystonia, epilepsy, and obsessive-compulsive disorder. Although effective, DBS is not currently a first-line treatment because of difficulties with precise placement, as well as risks of causing hemorrhagic stroke and other complications. Moreover, lengthy intraoperative electrical recordings and stimulus-response observations are currently required to position the probe. An embedded OCDR sensor may provide information on brain structures several millimeters ahead of the probe tip, enabling more precise, rapid, and safe placement. These improvements would allow DBS to benefit more patients. OCDR-guided deep-brain probes could also provide precise delivery of therapeutic vehicles in gene therapy, neurotransplantation, neuro-ablation, and pharmacologic treatments. The investigators include an original developer of OCDR and optical coherence tomography (OCT) in biomedical applications and an expert practitioner and developer of the DBS technique. The combined expertise of this team will take several promising approaches to using OCDR to identify brain tissue types (cortex, tracts, nuclei, and blood vessels) through a miniature probe. The following Specific Aims are proposed: Aim 1: Develop a combined OCDR/microelectrode brain probe. Aim 2: Distinguish brain tissue types using a dual-wavelength OCDR system that measures tissue reflectivity, attenuation, hydration, and birefringence. Aim 3: Detect blood vessels by Doppler shift and broadening of OCDR spectrum. Aim 4: Develop analysis & display software to identify tissues and guide probe advance. The OCDR brain probe technology will be validated in the laboratory and tested in a rat model. The practical knowledge gained in this pilot project will be used to develop a clinical OCDR-guided deep-brain probe for human DBS studies. Successful completion of this project would greatly benefit patients with many neurologic and psychiatric disorders.